In this study, I experimentally examined how the landscape matrix influenced the movement, oviposition behavior and spatial distribution of Anagrus columbi, a common egg parasitoid of the planthopper Prokelisia crocea. Both species exist among discrete patches of prairie cordgrass (Spartina pectinata), the sole host plant of P. crocea. Based on out-planted cordgrass pots bearing host eggs (to assess parasitism) or sticky leaves (traps for adult A. columbi), I found that the distribution of adult female A. columbi and pattern of ovipositions within a cordgrass patch were strongly matrix dependent. Female densities were 59% lower on the edge than interior of patches embedded in a mudflat matrix, but were evenly distributed within patches embedded in a matrix consisting of either native grasses or the exotic grass smooth brome (Bromus inermis). In contrast, parasitism was higher in the interior than edge for patches in all three matrix types. The lack of correspondence between A. columbi density and parasitism was attributed to differences in oviposition behavior: A. columbi parasitized 71% more hosts per-capita in the interior than edge for patches embedded in non-host grasses, but equal numbers on the edge and interior of patches embedded in mudflat. Matrix-dependent differences in the within-patch distribution and oviposition behavior of A. columbi can influence the distribution of parasitism risk and host-parasitoid stability at the patch level.
Matrix composition also affected the pattern of movement through the matrix and the colonization of nearby cordgrass patches. Anagrus columbi females emigrating from a mudflat-embedded patch were captured at very low, but constant, numbers with distance out into the matrix, suggesting that they were reluctant to enter or remain in the mudflat. In contrast, A. columbi females entering a non-host grass matrix had numbers that were high near the patch border and then declined exponentially with distance. These patterns of movement were likely responsible for the very different colonization rates for experimental patches embedded in different matrix types and located 3 m from a source patch of A. columbi. Patches embedded in brome were colonized at a rate that was 3.0 and 5.7 times higher than for patches in native grass or mudflat, respectively. Finally, based on a census of cordgrass patches spanning 5 generations, A. columbi densities and proportion of patches occupied generally increased with increasing host density, patch isolation and the proportion of the surrounding matrix that was mudflat. Patch size had no effect on the distribution of A. columbi. Overall, these data suggest that cordgrass patches in a non-host grass matrix, particularly smooth brome, have high connectivity relative to patches in a mudflat matrix. Changes in connectivity due to changes in matrix composition can significantly influence host-parasitoid persistence at the metapopulation level.
Key words: Anagrus, connectivity, diffusion, dispersal, egg parasitoid, edge effect, immigration, landscape matrix, metapopulation, patch dynamics, patch occupancy, planthoppers, Prokelisia, Spartina, tallgrass prairie.